1. Field of the Invention
The present invention relates to a semiconductor device that can communicate via radio waves.
2. Description of the Related Art
A technique to communicate signals by noncontact (RFID: Radio Frequency Identification) between a reader and a medium (ID tag) incorporating an antenna and an integrated circuit has become commonplace in various fields, and the further market expansion is expected as a new communication form. Although in many cases the shape of an ID tag for use in RFID is a card shape or a chip shape which is smaller than a card, various types of shapes are employed depending on the intended use of the ID tag.
With RFID, an ID tag and a reader can communicate with each other using radio waves. Specifically, radio waves radiated from the reader is converted into an electric signal in the antenna inside the ID tag, and the integrated circuit in the ID tag operates in response to the electric signal. Then, signals can be transmitted to the reader in noncontact by modulated radio waves being radiated from the antenna in accordance with the electric signals output from the integrated circuit.
Note that the ID tag is divided into two types: An ID tag with a built-in battery, and a batteryless ID tag. The Batteryless ID tag generates required electric power each time by converting radio waves from the reader into electrical energy. Therefore, when radiation of radio waves from the reader stops, supply of electrical energy to the integrated circuit which ID tag has also stops. On the other hand, the ID tag with a built-in battery ensures a certain level of electrical energy which is required for driving the integrated circuit by the battery incorporated in the ID tag.
Reference 1 (Japanese Published Patent Application No. 2005-316724) discloses a technique related to the ID tag with a built-in primary battery. Further, Reference 2 (Japanese Published Patent Application No. 2004-021612) discloses a technique related to an ID tag with a built-in secondary battery.